Optical system



Sepf- 19, 1939. L. T. SACHTLEBEN Z$73,266

OPTICAL SYSTEM Filed Jan. 29, 1958 LIGHT' Naz/7H .4576"2( 00a (MHG-E WELL) nnentor Lawrence 7;' 'aciz Zehen,

Patented Sept. 19, 1939 UNITED STATES oPTIcAL SYSTEM Lawrence T. Sachtlcben, Camden, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application January 29,

Claims.

This invention relates to optical systems such as are utilized in the recording of audio frequency or other electrical impulses, and has for its principal object the provision of an improved system 5 and method of operation whereby various types of distortion, heretofore encountered in the operation of such systems, are avoided.

It is customary in a conventional electrical impulse recorder of the galvanometer type to apply, light to the vibratable galvanometer reflector through a path which is displaced by l5 degrees to one side of the normal to the rotational axis of the reflector, and to receive thereflected .light through a path which is displaced to the opposite side of this normal by a similar angle of l5 degrees. Centered on the axis of the path through which light is applied to the reilector are a light aperture of triangular or other suitable shape and a lens through which the aperture is imaged' on the light slit. This aperture image is reiected through one or more lenses to a light slit across which it is vibrated in accordance with the electrical impulses applied to the operating coil of the galvanometer, and the impulse modulated light emerging from the slit is applied to a photographic record strip which is moved transversely of the slit. Under these conditions, the impulses are recorded on the stripV in the form of a variable area record track.

In order to avoid Various types of image distortion, it has been customary heretofore to mount the lens through which the aperture is imaged on thelight slit so that this lens is out of the path of the light received from the refiector. While this arrangement of the lens is satisfactory Where no restrictions are imposed on the dimensionsof the optical system, it is impractical in the case of a very compact optical system, for the reason that the aperture imaging lens must be mounted so close to the galva'nometer reiiector that both the incident and reected light pass through it.

'I'here is therefore encountered the problem of providing a lens which may be utilized in the 4 foregoing environment Without image bias, coma,

lchromatic diierence of magnification, spherical aberration, astigmatism, and other types of distortion. In accordance with the present invention, this difficulty is solved by the provision of an improved optical system including a'window lens which is meniscus in form with the concave side toward the reector, and is so designed that, when working at a one to one magnication, a field Whose center of curvature is at the reector will be imaged so that the primary image 1938, Serial No. 187,595

surface will also have its center of curvature at the mirror. As will be understood, the provision of such a lens is highly advantageous for the reason that it renders the aperture image free of coma and distortion andpermits lthe image to 5lk beaccurately located in a plane at right angles to the axis of the path along which light is re-y ceived from vthe reector so that it is at all times properly focused yat the light slit.

The invention will be better understood from vthe following description considered in connection with the accompanying drawing, and its scope is indicated by the appended claims.

Referring to the drawing,

Figure 1 illustrates a preferred embodiment of the invention, and

Figure 2 is an explanatory diagram relating to the/operation ci the arrangement of Fig. 1.

The system of Fig. 1 includes an aperture plate lil, a protective window l l for excluding dust and the like from the aperture, and a meniscus lens l2 and a mirror i3 through which the aperture of the plate il] is imaged on the light slit of the plate i6. The mirror i3 is arranged to be rotated or vibrated about the axis lil in response to the electrical impulses applied to its operating coil (not shown), Light from the source image produced on the mirror i3 is passed through the lens i2 and a lens i5 to an image of the aperture plate i@ at the light slit of a plate i6. The aperture of the plate l@ may be of trliangular or any other suitable shape such that the light emerging from the slit is modulated in accordance with the electrical impulses applied to the mirror operating coil and may be applied to a photographic recording surface moved at a constant rate of speed transversely of the slit.

Since suitable galvanometer constructions and lm moving mechanisms are well known and form no part of the present invention, they have been omitted from the drawing. It is also apparent that any suitable source of light may be provided for illuminatingy the aperture of the plate lli. How the various available lightv sources, galvanometers and record moving mech- 45 anisms may be applied in connection with the present invention is too well understood by those skilled in the art to require detailed consideration.

fThe exact design of the lens i2 is of great importance if the image of the aperture of the plate la at the light` slit is to be free of coma and distortion and is to lie in a plane parallel with the' light slit.V The symmetrical object and image, in? which B and B' are conjugate points, will in genvflo that, at unit :1.- 35' eral lie on curved surfaces either convex or concave toward the aperture. 'I'he shape or curvature of these symmetrical pairs of object and image surfaces may be adjusted by modifying the shape of the lens and, its distance from the aperture, thereby modifying the extent of the eccentric refraction of the oblique pencils or rays. In general, making the lenses more concave toward the aperture attens the two conjugate surfaces, while making the lenses less concave ln this Arespect makes the two surfaces more concave toward the aperture.

The symmetrical system of Fig. 1 consists of the lens I2; the mirror I3 which acts as the aperture, and the image of the lens produced by the mirror. By ,designing the shape of lens I2 so that the symmetrical surfaces containing the conjugate points B and B' are approximately spheres with their common'center of curvature at the mirror I3, the lens I2 and the mirror'y I3 can lbe rotated together through a limited'angle about the axis It through the center of the aperture without sensibly'changing the image of an object in the object surface. When this condition is met, the aperture is imaged at the light slit in a surface substantially normal to the axis of the reflected light. If the object and image surfaces are not centered at the mirror, then the image of the aperture at the light slit is not( in a surface normal to the axis of the reflected light, but is on a bias, with one side of the image in sharp focus at the light slit, while the other side is out of focus in the plane of the light slit.

The dimensions of one suitable optical system are for the most part apparent from the legends andnumerals applied to the various parts of Fig. 1. It will be noted that the distance between the aperture plate I0 and vthe mirror I 3 is 1.875 inches, and the distance between the mirror and the light slit is 2 inches. The aperture of the plate I 0 may be in the form of a triangle 0.530 inch wide by 0.300 inch high, the 0.530 winch dimension being in the plane of the drawing. The biconvex lens I5 may be'of crystal quartz having an index of refraction of 1.558, a thickness of 0.127 inch, and radii of curvature of 1.139 inches with the optical axis of the crystal parallel to the axis of the lens. The size of the aperture of the plate I0 is determined by theoverall magnication of its image in the plane of the light slit C, this value being about 0.980X when the effect of the lens I5 is included. The lens I2 may have a meniscus form, with the concave surface toward the mirror, the radius of curvature of the concave surface being 3.573 inches and the radius of'curvature of the convex surface being 0.697 inch when the refraction of the glass for the D line is 1.523. The free diameter of the lens may be 1/2 inch and the distance from the surface lof the mirror to the convex face of the lens maybe of the order'of 0.318 inch. The axis of the lens I2 may be tilted in a direction normal to the plane of the `two axes of the system which intersect at an angle of 30; through an angle such as 4 to 6, to prevent reflections from the surfaces of lens I2 from entering the light slit.

- Every part of the mirror It must of course receive light from every point of the aperture and must deliver it to the light slit at an angle of 30 degrees with the ams of' the aperture and without `vign'etting.

Fig. 2 illustrates in greater detail the characteristics of the lens I2. If this lens is so designed 'cation, an' object surface O of radius R centered at 'A is imaged upon the image surface I alsofcentered atv A, then atlthe point p (wherean auxiliary, axis pp', at an angle of 15 degrees with the optical axis OI, intersects the object surface), it is possible to place a plane object OO' and obtain at p' a rectilinear image z'i. Slight deviation from unit magnification does not appreciably affect the quality or effective llatness of theimage. Ideally, the Petzval condition should be met by coincidence of the primary and secondary image surfaces in the surface I. It will, of course, be understood that the lens- I2 may be formed of a single piece or may be formed of separate parts having different characteristics depending on the extent to which the various forms of image distortion are to be eliminated..

I claimas my invention:

1. An optical system including means forming a light aperture and a light slit centerednn different axes, a reflector arranged at the in rception of said axes to vibrate an image of said aperture transversely of said slit, and a lens arranged to intercept both of said axes for centering substantially at said mirror both the lleld of said aperture and the field of the aperture image at said slit, saidvlens having such rad of curvature l vfocal length, said ileld` having a curvature of a radius equal to the focal length of the lens.

2. An optical system including means forming l a light aperture and a light slit centered on different axes, a reflector arranged at the interception of said axes to vibrate an image of said aperture-transversely of said slit, and a meniscus lens arranged in said axes to center substantially at said mirror both the field of said aperture and the field of the aperture image at said slit, said lens having such radii of curvature as to have a spherical eld for both the incident and reilected rays at a distance equal toits own focal length, said ileld having a curvature of a radius equal to the focal length of the lens.

3. An optical system including means forming a light aperture and a light slit centered on different axes, a reflector arranged at the interception of said axes to vibrate an image of said aperture transversely of said slit, and a meniscus lens arranged in said axes with its concave side facing vsaid mirror for centering substantially at said said eld having a curvature of radius equal to the focal length of the lens.

4. An optical system including means forming a lightapertureand a light slit centered on different axes, a reflector arranged at the intercep.

tion of said axes to vibrate an image of said aperture transversely of said slit, and a meniscus lens arranged insaid axes with its concave side facing said mirror and centering substantially at said mirror both the iieldof said aperture and the field of the primary aperture image at said slit, said lens having such radii of curvature as to have a spherical field for both the incident and reected rays at a distance equal to its own focal length, said field having a curvature of a radius equal to the focal length of the lens.

5. An optical system including means forming a lightaperture and a light slit centered on different axes, a reector arranged at the interception of 'said axes to vibrate an image' of vsaid aperture transversely of said slit, and a lens arranged the surfaces of said lens from entering said slit, to intercept both of said axes for centering subsaid lens having such radii of curvature as to stantially at said mirror both the eld of said have a spherical eld for both the incident and aperture and the field of the aperture image at r'eiected rays at a distance equal toits own focal `5L said slit, the optical axis of said lens being dis`- length, said eld having a curvature of a radius 5 placed in a direction normal to the plane of said equal to the focal length of the lens. intersecting axes to prevent light reflected from LAWRENCE T. SACHTLEBEN. 

